A magnetic random access memory (MRAM) is a non-volatile memory which uses multiple layers of magnetoresistive materials as memory storage elements. A typical MRAM array includes a number of conductive bit or digit lines intersected by conductive word lines. At each intersection, a magnetoresistive element, commonly referred to as a magnetic tunnel junction (MTJ) memory element is formed. Each magnetic memory element includes a first pinned ferromagnetic material layer (where the magnetic field is fixed) and a second free ferromagnetic material layer (where the magnetic field can, by programming, assume two different orientations). The two ferromagnetic layers are separated by a non-magnetic layer, commonly referred to as the tunnel barrier layer. The direction of the magnetic vectors in the free and pinned layers, determines the stored state of the magnetic memory element. As such, the magnetic memory element has two stable magnetic states. In one magnetic direction of the free layer the memory element has a high resistance defined as one logic value, e.g. “0” and in the other magnetic direction of the free layer the memory element has a low resistance, defined as the other logic value, e.g. “1.” The stored state of the memory element is generally read by providing a sense current through the magnetic memory element to determine its resistance.
FIG. 1 illustrates an exemplary conventional MRAM structure including three magnetic memory elements 22, having respective associated conductive lines 18 and 28 which may serve as word lines and bit lines. Conductive lines 18, typically formed of copper, are formed in an insulating layer 16 formed over under-layers 14 of an integrated circuit (IC) substrate 10. Under-layers 14 may include, for example portions of integrated circuitry, such as CMOS circuitry. A pinned layer 20 is provided over the conductive lines 18. A tunnel barrier layer 24 is provided over the pinned layer 20. The tunnel barrier layer 24 is generally formed of aluminum oxide. A free layer 26 is provided over the tunnel barrier layer 24. Another conductive line 28 is formed over the free layer 26.
Generally, the resistance across the magnetic memory element 22 is dependent on both the surface area and thickness of the tunnel barrier layer 24. However, in the fabrication of conventional MRAM structures, the surface area of the tunnel barrier layer 24 is equivalent to the surface area of the free layer 26. Consequently, controlling the resistance of the magnetic memory element 22 is limited to adjusting the thickness of the tunnel barrier layer 24. Accordingly, conventional methods used for reducing the resistance of the tunnel barrier layer, require making the tunnel barrier layer as thin as possible, which is generally between about 5 Angstroms to about 20 Angstroms. However, having such a thin tunnel barrier layer 24 does not provide adequate protection against a short 23 being formed across the tunnel barrier layer 24 at edges of the magnetic memory element 22, which may occur during the etch patterning of the magnetic memory element 22. A shorted magnetic memory element 22 can not store a logic value of a “1” or “0” as different resistance values.
Accordingly, there is a need for an improved method for forming magnetic memory elements having greater reliability against shorting. There is also a need for a method for forming a magnetic memory element where the resistance across the tunnel barrier layer can be set independently of the surface area of a corresponding free layer.